The invention relates to a semiconductor device for emitting electrons, comprising a semiconductor body with at least one pn junction between an n-type region adjacent to a main surface and a p-type region, in which electrons emitted from the semiconductor body are generated by applying a voltage in the reverse direction across the pn junction, a first part of the pn junction, at the location of an emissive part of the pn junction, extending substantially parallel to the main surface and locally having a lower breakdown voltage than a second part of the pn junction, the first part being separated from the main surface by a thin n-type layer having such a thickness and doping that the depletion zone does not extend as far as the main surface at the breakdown voltage but remains separated from said main surface by a surface layer which is sufficiently thin to pass generated electrons.
A semiconductor device of this type is used in an electron tube which may be used as a display tube or a camera tube, but may be alternatively adapted, for example, for electrolithographic applications or electron microscopy.
The invention also relates to an electron tube comprising such a semiconductor device.
In this description, pn junction is not only understood to mean a pure pn junction but any junction structure between a p-type region and the n-type region; it may therefore be composed of a plurality of layers such as, for example a pin diode, a p.pi..nu.n diode, etc. or, for example two pn junctions in series.
A semiconductor device of the type described above is disclosed in U.S. Pat. No. 5,604,355. In the semiconductor device, which is a "cold cathode", a pn junction is reverse biased in such a way that there is avalanche multiplication of charge carriers. Some electrons may then acquire as much kinetic energy as is necessary for exceeding the electron work function. The emission of these electrons is simplified by providing the semiconductor device with acceleration electrodes or gate electrodes on an insulating layer located on the main surface, which insulating layer leaves an aperture at the location of the emissive region. Emission is still further simplified by providing the semiconductor surface at the location of the emissive region with a material reducing the work function such as, for example cesium.
If such a cathode is built into an electron tube, problems occur in the further manufacturing process. During the process, which is known as spot-knocking, a number of grids in the tube acquire a high to very high mutual voltage difference (30 kV to 100 kV). During this spot-knocking operation, flashovers are produced so that a high-voltage difference (10 kV to 30 kV) occurs between the grid located closest to the cathode and the cathode itself. Such a flashover may also occur during normal use.
The connection wires of the (p-type) substrate, the n-type region as well as the gate electrodes cannot, however, be considered as purely ohmic connections but have a given inductance. This results in a large voltage difference between the substrate and the gate electrode due to capacitive crosstalk between said grid and, for example this substrate. This voltage difference is also dependent on the inductances of the connection wires, the resistance of the (semiconductor) material and the duration of the flashover. As a result, electron tubes comprising this type of cold cathode are often rejected, notably during the spot-knocking process, but also during normal operation.
To prevent destructive breakdown of the insulating layer between the gate electrode and the subjacent substrate, the device disclosed in U.S. Pat. No. 5,604,355 is provided with extra semiconductor structures which convey current during the occurrence of flashovers. However, a possible increase of the voltage of the n-type region, for example via the associated bond-flap, still leads to high currents through the pn junction and through the regions constituted by the n-type surface regions for removing non-emitted electrons (draining channels) while the thin n-type layer is quickly damaged.